Fission is important in the philosophy of identity – especially of personal identity – in that it provides many thought-experiments4 (TEs), and some real-life examples, that test out the various theories.

An alternative account is to claim that the two half-brains always were separate persons (and Puccetti has maintained that they are separate persons, in all of us, even prior to the commissurotomy11 in this TE), in a way slightly different from the usual Lewis12 view of non-identical spatially-coincident13individuals14 (because the hemispheres aren’t spatially coincident, though the shared body15 is).

We need to consider how the original person16 was unified17. We can press the realism of the thought-experiment by asking how important are the spinal cord and PNS generally to the psychological integrity of the human organism18? The case of dicephalus19 twins may be relevant – where the functions of walking and even typing seem to be carried out perfectly adequately despite the coordinated limbs being controlled by different brains.

We also need to consider whether the two half-brains continue to constitute a single scattered20 person, just parked in separate bodies. A single embodiment is important because it ensures synchronisation of experience, and external communication between the hemispheres (in the absence of the usual internal communication across the corpus callosum). Presumably, this could be achieved in other ways.

We can imagine a BIV21 linked by radio transmitters/receivers to a remote body – the brain is part of the body – so a single physical thing can be spatially discontinuous. Why, if A fissions into B and C, can’t we consider B & C to be parts of the same person? They could fight / argue … but so can someone in two minds about things. What if one killed the other? They would have different perceptual experiences, but so (presumably) does a chameleon, with its eyes pointing in different directions (and sheep and other herbivores, and fish, with eyes on the sides of their heads).

I need to consider in detail what is supposed to be going on in fission – ie. press the thought experiment: there needs to be segregation / redundancy prior to separation – this can happen over time (or we would have plain duplication22). At some point the person will split, with incommunicable consciousnesses23 (cf. Locke24’s day-person and night-person).